There has been an overall trend in recent years towards lower exhaust temperatures in many applications, both industrial and mobile, that use urea-SCR emission controls systems. These reductions in exhaust temperatures are driven by variety of reasons that includes improvements in fuel efficiency, engine operation modes, among others.
The use of lower exhaust temperatures poses significant challenges to current SCR technology due to the presence of SOx species in the flue gases primarily derived from some fuel sources such as oil and coal. The combination of low temperature and high sulphur content will lead to formation of ammonium bisulphates (ABS) that will deactivate the SCR catalyst by masking effects. To prevent the formation of ABS, it is required to either increase the operating temperature, so that ABS formation does not take place, or scrub the SOx species prior to the SCR unit. Both of these procedures are either technically unfeasible or economically undesirable. Another solution to this problem is to operate engines using low or ultra-low sulphur fuels. However; this carries with it an economic penalty in that the cost of low or ultra-low sulphur fuels are typically much greater than the cost of sulphur rich fuel. In addition, the availability of such low sulphur fuels may also be limited.
It is widely known in the field of flue gas catalysis that the SCR reaction can be promoted by the presence of NO2. However, it has not been possible to find an economical solution to generate NO2 for non-automotive applications due to the volume of catalyst required, the associated cost of the catalyst and the variations in the costs and compositions of various fuels. The most common method of generating NO2 would be via an oxidation catalyst that is generally in some systems to abate carbon monoxide (CO) and hydrocarbons (HCs). However, these oxidation catalysts are also highly sensitive to SOx species and undergo rapid deactivation. Thus there is a need for a system that allows for the continuous operation of a urea-SCR system at low temperatures that is operated in the presence of high SOx containing flue gas.